Worlds in Collision

Artist’s conception of the planetary collision in the BD +20 307 binary system (Lynette Cook)
Full resolution 3600 x 2400 JPG file (5 MB) available here.
Reprint of paper published in Astrophysical Journal available here.

(September 22, 2008, Nashville, TN / Los Angeles, CA): Two terrestrial
planets in orbit around a mature sun-like star recently suffered a violent
collision, astronomers at University of California Los Angeles, Tennessee
State University, and California Institute of Technology will report in a
December issue of the Astrophysical Journal, the premier journal of astronomy
and astrophysics.

“It’s as if Earth and Venus collided with each other,” said Benjamin
Zuckerman, UCLA professor of physics and astronomy and a co-author on the
paper. “Astronomers have never seen anything like this before; apparently
major, catastrophic, collisions can take place in a fully mature planetary
system.”

“If any life was present on either planet, the massive collision would
have wiped out everything in a matter of minutes: the ultimate extinction
event,” said coauthor Gregory Henry, an astronomer at Tennessee State
University. “A massive disk of infrared-emitting dust circling the star
provides silent testimony to this sad fate,” said Henry.

Zuckerman, Henry, and Michael Muno (a former Caltech astronomer) were
studying a star known as BD +20 307, which is surrounded by a shocking one
million times more dust than is orbiting around our sun. The star is
located about 300 light-years from Earth in the constellation Aries. The
astronomers gathered X-ray data with the orbiting Chandra X-ray Observatory
and brightness data from one of TSU’s automated telescopes in southern
Arizona, hoping to measure the age of the star.

“We expected to find that BD +20 307 was relatively young, a few hundred
million years old at most, with the massive dust ring signaling the final
stages in the formation of the star’s planetary system,” said Muno.

However, those expectations were shattered when Carnegie Institution of
Washington astronomer Alycia Weinberger announced in the May 20th issue of
the Astrophysical Journal that BD +20 307 is a close binary star.

“That discovery radically revised the interpretation of the data and
transformed the star into a unique and intriguing system,” said TSU
astronomer Francis Fekel who, along with TSU’s Michael Williamson, was
asked to provide spectroscopic data from another TSU automated telescope
in Arizona to assist in comprehending this exceptional binary system.

The new spectroscopic data confirmed that BD +20 307 is composed of two
stars, both very similar in mass, temperature, and size to our own Sun.
They orbit about their common center of mass every 3.42 days.

“The patterns of element abundances in the stars show that they are much
older than a few hundred million years, as originally thought,” said Fekel.
“Instead, the binary system appears to have an age of several billion years,
comparable to our solar system.”

“The planetary collision in BD +20 307 was not observed directly but,
rather, was inferred from the extraordinary quantity of dust particles that
orbit the binary pair at about the same distance as Earth and Venus are from
our Sun,” Henry said. “If this dust does indeed point to the presence of
terrestrial planets, then this represents the first known example of planets
of any mass in orbit around a close binary star.”

Zuckerman and colleagues first reported in the journal Nature in July 2005
that BD +20 307, then still thought to be a single star, is surrounded by
more warm orbiting dust than any other sun-like star known to astronomers.
The dust is orbiting the binary system in close, where Earth-like planets
are most likely to be and where dust typically cannot survive long. Small
dust particles get pushed away by stellar radiation, while larger pieces get
reduced to dust in collisions within the disk and are then whisked away.
Thus, the dust-forming collision near BD +20 307 must have taken place rather
recently, probably within the past few hundred thousand years and perhaps
much more recently, the astronomers said.

“This poses two very interesting questions,” Fekel said. “How do planetary
orbits become destabilized in such an old, mature system? Could such a
collision happen in our own solar system?”

"The stability of planetary orbits in our own solar system has been
considered for nearly two decades by astronomer Jacques Laskar in France and,
more recently, by Konstantin Batygin and Greg Laughlin in the USA," Henry
noted. "Their computer models predict planetary motions into the distant
future and they find a small probability for collisions of Mercury with
Earth or Venus sometime in the next billion years or more. The small
probability of this happening is probably related to the rarity of very
dusty planetary systems like BD +20 307."

“There is no question, however,” said Zuckerman, “that major collisions
have occurred in our solar system’s past. Many astronomers believe our moon
was formed from the grazing collision of two planetary embryos — the young
Earth and a body about the size of Mars — a crash that created tremendous
debris some of which condensed to form the moon, and some of which went into
orbit around the young sun. By contrast with the massive crash in the
BD +20 307 system, the collision of an asteroid with Earth 65 million years
ago, the most favored explanation for the final demise of the dinosaurs, was
a mere pipsqueak.”

In their 1932 novel “When Worlds Collide,” science fiction writers Philip
Wylie and Edwin Balmer destroyed the Earth by a collision with a planet of a
passing star. The 1951 classic movie, based on the novel, began a long line
of adventure stories of space-rocks apocalyptically plowing into Earth.
“But,” Zuckerman noted, “there is no evidence near BD +20 307 of any such
passing star.”

This research is federally funded by the National Science Foundation and
NASA and also by Tennessee State University and the State of Tennessee
through its Centers of Excellence program.
The Astrophysical Journal is published by the American Astronomical Society.

TSU is a comprehensive, urban, coeducational land-grant university,
offering 45 bachelor's degrees, 24 master's degrees, and six doctoral
programs. Its diverse student population totals about 9000 students, who
come from 46 states and 45 countries. TSU has been listed for 11 consecutive
years in the U.S. News & World Report "Guide to America's Best Colleges."

UCLA is California’s largest university, with an enrollment of nearly
37,000 undergraduate and graduate students. The UCLA College of Letters
and Science and the university’s 11 professional schools feature renowned
faculty and offer more than 300 degree programs and majors. UCLA is a
national and international leader in the breadth and quality of its academic,
research, health care, cultural, continuing education and athletic programs.
Four alumni and five faculty have been awarded the Nobel Prize.